Scaling dielectric layers, including silicon dioxide (SiO2) beyond 2 nm gives rise to large leakage current due to direct tunneling. Thus, alternative high-k dielectrics have been proposed. Generally, “high-K” refers to dielectric constants greater than that of SiO2 (K˜3.9). As used herein, “high-K” will refer to dielectric constants greater than 15, while “medium-K” will refer to dielectric constants between approximately 4-15.
The common approach has involved amorphous materials with higher dielectric constants, such as hafnium or zirconium oxides (K˜20-25) and their silicates (K˜10-14). The former group (oxides) show poor thermal stability and undergo reinsulatorlization at modest temperature (>800 C). The later (silicates) achieve higher thermal stability at the expense of lower dielectric constants. Both groups, in general, when used as a gate dielectric directly on a silicon substrate, exhibit high interface state density and consequently severe mobility degradation for a field effect transistor (FET) device. Additionally, oxygen-vacancy induced defects create a high density of shallow traps introducing threshold instability and reliability concerns.